Scientists find huge reduction in African dust plume led to more Saharan monsoons 11,000 years ago

Last year Ralph Ellis proposed a ‘dust theory of ice ages’ which we featured at the Talkshop. This research looks interesting in that context, and in its own right too.

Every year, trade winds over the Sahara Desert sweep up huge plumes of mineral dust, transporting hundreds of teragrams—enough to fill 10 million dump trucks—across North Africa and over the Atlantic Ocean.

This dust can be blown for thousands of kilometers and settle in places as far away as Florida and the Bahamas. The Sahara is the largest source of windblown dust to the Earth’s atmosphere.

But researchers from MIT, Yale University, and elsewhere now report that the African plume was far less dusty between 5,000 and 11,000 years ago, containing only half the amount of dust that is transported today.

In a paper published today in Science Advances, the researchers have reconstructed the African dust plume over the last 23,000 years and observed a dramatic reduction in dust beginning around 11,000 years ago. They say this weakened plume may have allowed more sunlight to reach the ocean, increasing its temperature by 0.15 degrees Celsius—a small but significant spike that likely helped whip up monsoons over North Africa, where climate at the time was far more temperate and hospitable than it is today.

“In the tropical ocean, fractions of a degree can cause big differences in precipitation patterns and winds,” says co-author David McGee, the Kerr-McGee Career Development Assistant Professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “It does seem like dust variations may have large enough effects that it’s important to know how big those impacts were in past and future climates.”

Talkshop note: the above is just the first part of the Phys.org report. The report concludes:

Dust’s climate role

McGee and his colleagues obtained sediment core samples from the Bahamas that were collected in the 1980s by scientists from the Woods Hole Oceanographic Institution. They brought the samples back to the lab and analyzed their chemical composition, including isotopes of thorium—an element that exists in windblown dust worldwide, at known concentrations.

They determined how much dust was in each sediment layer by measuring the primary isotope of thorium, and determined how fast it was accumulating by measuring the amount of a rare thorium isotope in each layer.

In this way, the team analyzed sediment layers from the last 23,000 years, and showed that around 16,000 years ago, toward the end of the last ice age, the dust plume was at its highest, lofting at least twice the amount of dust over the Atlantic, compared to today. However, between 5,000 and 11,000 years ago, this plume weakened significantly, with just half the amount of today’s windblown dust.

Colleagues at Yale University then plugged their estimates into a climate model to see how such changes in the African dust plume would affect both ocean temperatures in the North Atlantic and overall climate in North Africa. The simulations showed that a drop in long-range windblown dust would raise sea surface temperatures by 0.15 degrees Celsius, drawing more water vapor over the Sahara, which would have helped to drive more intense monsoon rains in the region.

“The modeling showed that if dust had even relatively small impacts on sea surface temperatures, this could have pronounced impacts on precipitation and winds both in the north Atlantic and over North Africa,” McGee says. Noting that the next key step is to reduce uncertainties in the modeling of dust’s climate impacts, he adds: “We’re not saying, the expansion of monsoon rains into the Sahara was caused solely by dust impacts. We’re saying we need to figure out how big those dust impacts are, to understand both past and future climates.”